Projection apparatus



Patented spr.,1s,g194z UNITED l STATES PATEN T OFFICE PROJECTION APPARATUS f David W. Epstein, Merchantville, N. J., asslgnor to Radio Corporation of America, a corporation ot Delaware applicati@ August-17, 1940, serial No. 353,003

,(cl. 17a-1.5)

7 Claims.

tern of the above-mentioned type. In the television projector disclosed by Landis, the cathode ray receiver tube is-located in front of a, spherical mirror and at such a point that the spherical mirror in cooperation with a correcting plate projects an enlarged image of the reproduced television picture on a screen. With this arrangement, the cathode ray tube and its associated parts, such as the deflecting coil and the light rays reflected from the spherical mirror.

the event oi failure of the cathode ray tube or other image source.

In accordance with a preferred embodiment of l my invention, the optical system consists of a spherical mirror having an opening in the center thereof, a correcting plate positioned in front "tube supports, are in the path, of some of the In some designs, the relative dimensions of the optical system andthe cathode ray tube and associated parts are such that the blocking of the light is negligible and has no appreciable eiect on the projected picture. In other cases, however, especially where it is desired to employ an optical system of small dimensions, which in turn `requires that the diameter ofthe cathode ray tube be small, it is found that the deflecting yoke and/or the tube supports block so much of the correcting plate and thus'the light emanating from the edge portions of the picture that the projected picture appears on the projection screen with insumcient illumination at the edges of the picture. In still other cases, the tube or the tube and its associated parts may completely block the light reected from the spherical mirror.

Also, in 4a system as described in the abovementioned Landis patent, it is diiicult, if not impossible, to replace the cathode ray tube, in event of tube failure, with so little delay that there is substantially no interruption of the program.

One of the objects of my invention is to provide an improved optical system of the abovedescribed type.

A further object of my invention is' to provide an improved optical system of the above-mentioned type wherein in certain cases blocking of the light reflected from` the mirror is substantially reduced.

A still further object of my invention is to provide an improved projection apparatus of the above-described type which may be operated with substantially no interruption of the program, in 55 indicated at' lll, and the resulting picture SignalV of the spherical mirror to correct for spherical aberration, and a plane mirror located between the spherical mirror and the correcting plate with its reiiecting surface facing the opening in -a spherical mirror whereby the cathode ray tube or other image-producing source maybe positioned with its light-obstructing parts at thel rear of the" Spherical mirror.

In the embodiment of my invention in which Vthe projection apparatus is so designed that there will be substantially no program interruption in the event of projection tube failure, the projection system includes a cathode ray tube located atthe rear of the spherical mirror, as above described, and a second cathode ray tube located in front of the spherical mirror. In this application of the invention where the two projection tubes are employed, the relative dimensions of the optical system of the cathode ray tubes and their associated parts may be similar to those described in the above-identied Landis patent, whereby there is no appreciable blocking of the light by the cathode ray tube and associated parts located in front of the spherical mirror. With this arrangement, only one projection tube is in use at any one time, while the other projection tube is maintained ready for instant use but with no picture thereon.

The invention will be better understood from the following description taken in connection with the accompanying drawing, in which:

Figure l is a view showing an embodiment of my invention wherein a spare tube for the optical system may be switched in instantly in the event of failure of the projection tube being used;

Figure 2 is aview showing my invention applied to a television projector comprising an optical system of small size; and

Figure 3 is a view showing my improved optical system employed as a camera in combination with a cathode ray pickup tube of the type surrounded by a focusing coil.

Referring to Fig. 1, my invention is shown ap- .correcting plate I1.

supplied through a video amplifier II to a' pair of cathode ray tubes I2 and I3. i

Ii the tube I3 is in operation, the reproduced image appearing on the end thereof is projected upon ascreen indicated at Il by means of the optical system described in the above-mentioned Landis patent, this optical system consisting of a spherical mirror IB and a spherical aberration If the tube I2 is in operation, the reproduced picture appearing on the end thereof is projected upon the screen. I4 by means of my improved optical system consisting of a plane mirror I8, the spherical mirror I6, and the correcting plate During a program, the tubes I2 and I3 are maintained in condition for instant operation. They may be heid in thiscondition by applying all operating voltages thereto and' by applying the horizontal and vertical deiiecting waves to the deecting yokes I9 and 2l of the two tubes from the deecbion circuits indicated at v22. The spare tube may be rendered ineffective to. reproduce a picture by applying to its control grida negative voltage vsumcient to. block the scanning beam. This condition is indicated for the tube Il. in Fig. l, where a switch 23 is positioned to apply a high negative bias tothe control grid of tube I2 from a biasing source 2i.

The cathode ray tube I3, on the other hand, has a picture appearing thereon, if there is an incoming picture, since its associated switch 28 Vis positioned to apply normal operating bias to the control grid of the tube I3 from the biasing source 21. vAs indicated by the dotted line 25, the switches 23 and 26 may be gauged so that one switch is closed when the other one is opened.

In the example illustrated, the optical system isl mounted inside a cylindrical casing 28, which is open at both ends. As indicated on the drawing, the back side of the mirror I-B should be nonreecting.

The improved optical system per se will bedescribed in more detail with respect to Fig. 2. In thisiigure, the optical system consists of a spherical mirror 3|, a piane mirror 32 and a correcting plate 33. This particular system is one of comparatively small dimensions which is designed for use with a cathode ray projection tube of small diameter, such as the tube of 2-inch diameter indicated at Y345, the correcting plate 33 being 8 inches in diameter, for example. The deiiecting yoke for the y`cathode ray tube is indicated at '36. It will 'be apparent that, if the cathode ray tube 34 with its deiiecting yoke were positioned in front of the spherical mirror 3 I, the deiiecting yoke 36 would block a substantial part of the light reflected from the spherical mirror. Therefore, so much of the light from the picture edges would be blocked that the projected picture would have sumcient brightness only in the central region of the screen. Also, supporting means must be provided for the projection tube, such means being indicated at 31 and 38, and, in a system having comparatively small dimensions, the tube-supporting means will block a large part of the light if located in front of the spherical mirror.

The shape and relative size of the plane mirror 32 will depend upon the design of the particular projector. Ity may be noted, however, that it may be either circular or rectangular in shape and may have a diameter or transverse dimension about twice the diameter of the end of the cathode ray tube upon which the picture appears.

,television camera or pick-up unit which includes a cathode ray pick-up tube BI of the low velocity beam type surrounded by a focusing coil 42 and a deflecting'coil 3. The tube 4I includes a semitransparent mosaic I4 upon which is formed an image of the scene to be transmitted.

My invention permits the use of a very fast and highly corrected optical system with the abovedescribed type of low-velocity beam tube in spite ofthe comparatively large diameters of the coils 'd2 and 43. This optical system comprises a spher- 'ical mirror 46, a correcting plate 41, and a plane mirror e8, the design of the system being the same as previously described.

The optical system in combination with two cathode ray tubes, that is, the combination illustrated in Fig. 1, may be utilized for the reproduction of color pictures, if desired. It will be under.

stood that when the system is utilized for this purpose, the two cathode ray tubes are in operation simultaneously to project two superimposed images, one image being of one color and the other image being of a different color. As is well known in the art, the different colors for the two projection tubes may be obtained either by the use of color screens positioned in the front of cathode ray tubes or by means of iiuorescent material selected to give light of the desired color.

From the foregoing description, it will be apparent that various other modications may be made in my invention without departing from the spirit and scope thereof.

I claim as my invention:

l. For use in a television system, an optical system comprising a spherical mirror having an opening or window in the center thereof, a plane mirror having its reflecting surface facing said spherical mirror and located between said spherical mirror and its nearest conjugate focus, and a correcting plate located in the region of the center of curvature of said spherical mirror and shaped or figured to correct for spherical aberration.

2. In combination in a television system, an optical system comprising a spherical mirror having an opening or window in the center thereof, a plane mirror having its reflecting surface facing said spherical mirror and located between said spherical mirror and its nearest conjugate focus, a correcting plate located in the region of the center of curvature of said spherical mirror and shaped or figured to correct for spherical aberration, and an image source positioned opposite the reecting surface of said plane mirror and so located that effectively it is substantially at the said nearest conjugate focus of the spherical mirror. v

'3. Forvuse in a television system, a projection system comprising a spherical mirror having an opening or window in the center thereof, a plane mirror having its reflecting surface facing said spherical mirror and located between said spherical mirror and its nearest conjugate focus, a correcting plate located in the region of the cen- .asA

ter of curvature. of said spherical mirror and shaped or gured to correct for spherical aberration, means for providing an image located opposite the reflecting surface of said plane mirror and so positioned as to project said image upon a surface at the other conjugate focus of the optical system, and means for providing anv image located substantially at the said nearest conjugate focus whereby said last image will be projected upon said last surface.

4. A cathode ray tube projection system comprising a spherical mirror having an opening or window in the center thereof, a plane mirror having its reflecting surface facing said spherical mirror and located between said spherical mirror and its nearest conjugate focus, a correcting plate located in the region of thel center of curvature of said spherical mirror and shaped or figured to correct for spherical aberration, a. cathode ray tube having a screen structure upon which a picture may be reproduced, said screen structure being located opposite the reflecting surface of said plane mirror and so positioned as to project an image upon a surface at the other conjugate focus of the optical system, and a second cathode ray tube having a screen structure upon which a picture may be reproduced, said last screen structure being-located substantially at the said nearest conjugate focus whereby an image appearing thereon `will be projected to form an image upon said last surface.

5. 'I'he invention according toclaim 4 wherein means is provided for making only one of said cathode ray tubes effective to project a picture at a given time.

6. The invention according to claim 4 wherein means is provided for maintaining said two cathode ray tubes in operative condition for projecting a picture, and means for blocking the cathode ray of one of said tubes during the time the other of said tubes is projecting a picture.

'7. An optical system for the projection and enlarging of an image on a cathode ray tube, said system comprising a cathode ray tube adapted to emit light for producing a bi-dimensional optical image of finite area substantially centered on the optical axis of the system, a plane mirror positioned to reflect light from saidcathode ray tube image, a' light reflecting surface of revolution having its concave reflecting surface positioned facing said plane mirror to receive reflected light therefrom, an aspherical zone plate positioned to receive the light reflected from said concave surface, said zone plate'being arranged external to the light path from the cathode ray tube image to the concave reflecting surface and axially aligned with each and adapted to correct the spherical aberrations introduced into the re- .flected light rays by the concave reflecting surface whereby a sharply focused enlargement of the original nite dimension cathode ray tube image is developed at a plane located at a finite distance from the aspherical zone plate.

DAVID W. EPSTEIN. 

